Astronomers have captured a cataclysmic stellar explosion known as a gamma-ray burst, or GRB, and studied its enduring “afterglow”, confirming predictions made by University of Bath scientists and solving one of the puzzles about the physics of the universe’s biggest explosions.

In the blink of an eye, a ginormous star more than two billion light-years away lost a million-year-long fight against gravity and collapsed, triggering a supernova and forming a black hole at its center.

An artist’s impression of the reverse shock echoing back though the jets of the gamma-ray burst. Image credit: NRAO, AUI, NSF, S. Dagnello

This newborn black hole belched a fleeting yet astonishingly intense flash of gamma rays toward Earth, where it was detected by NASA’s Neil Gehrels Swift Observatory on 19 December 2016.

While the gamma rays from the burst disappeared from view after just seven seconds, the dying embers of the explosion, the afterglow, produced light at longer wavelengths of light — including X-ray, optical, and radio waves – that continued to shine for several weeks. This allowed astronomers to study the aftermath of this fantastically energetic event, known as GRB 161219B, with an impressive suite of ground-based observatories.

In particular the unique capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile enabled an international team of astronomers from the UK, US, Italy, Japan, Sweden and Germany to make an extended study of this explosion at millimeter wavelengths, gaining new insights into this spectacular GRB and the size and composition of its powerful jets.

They were seeking to confirm predictions made by astrophysicists led by the University of Bath’s Professor Carole Mundell. Her team predicted that bright flashes of radio waves should be produced by the aftermath of GRBs or so-called “reverse shocks”.

Reverse shocks occur when material blasted away from a GRB by its jets runs into a surrounding layer of dust and gas – the debris sloughed off from the star as it approached the explosive end of its life. This encounter slows down the escaping material, sending a shockwave back down the jet, which it was thought would create intense flashes of visible light lasting just few seconds. Mundell’s team developed novel instruments to search for and measure the properties of these optical flashes with autonomous robotic telescopes.

The puzzle was that only a small percentage of observed GRBs showed these bright visible flashes shortly after the explosion. The mystery of the missing reverse shocks motivated Mundell’s team to develop a theoretical framework that predicted scientists should instead search at wavelengths longer than visible light – infrared or radio wavelengths were a good bet, if the team’s hunches on the physics of jets physics and role of magnetic fields in accelerating and focusing the jets were correct.

The new ALMA results published in Astrophysical Journal are an exciting confirmation that these reverses shock can be readily detected –the light from this GRB lasted around a day, peaking in the millimeter band of light wavelength.

Professor Mundell, a co-author in the study, said: “For decades astronomers thought this reverse shock would produce a bright flash of optical light, which has so far been really hard to find despite careful searches with fast-moving robotic telescopes that respond automatically within minutes to alerts from satellites when they discover a new GRB. We suggested a model – the so-called low-frequency model – in which the conditions in the ejected plasma meant that for many GRBs, the reverse-shock flash was producing light at longer wavelengths.

“Our ALMA observations show that astronomers may have been looking in the wrong place, and that millimeter observations are our best hope of catching these cosmic fireworks. The phenomenal sensitivity of the ALMA facility and the new ability to respond to transient alerts is opening a new window on the dynamic universe.”

Lead author Dr Tanmoy Laskar, a Jansky Postdoctoral Fellow of the National Radio Astronomy Observatory in Berkeley, California, who will join Bath’s astrophysics group in the autumn, said: “Since ALMA sees in millimeter-wavelength light, which carries information on how the jets interact with the surrounding dust and gas, it is a powerful probe of these violent cosmic explosions. With our current understanding of GRBs, we would normally expect a reverse shock to last on a few seconds to a minute at most. This one lasted a good portion of an entire day,

“The ALMA observations we present suggest that low density environments are essential for producing reverse shock emission, which may explain why such signatures are so rare, confirming theoretical suggestions previously put forth by Prof Mundell and the astrophysics group at Bath.”

While the millimeter light seen by ALMA was created by the reverse shock, the X-ray and optical light seen by Swift and ground-based observatories came from the blast wave shock riding ahead of the jet.

“What was unique about this event,” Laskar added, “is that as the reverse shock entered the jet, it slowly but continuously transferred the jet’s energy into the forward-moving blast wave, causing the optical and X-ray light to fade much slower than expected. Astronomers have always puzzled where this extra energy in the blast wave comes from. Thank to ALMA, we know this energy – up to 85 percent of the total in the case of GRB 161219B – is hidden in slow-moving material within the jet itself.”

These observations enabled the astronomers to produce ALMA’s first-ever time-lapse movie of a cosmic explosion, which revealed the surprisingly long-lasting reverse shockwave from the explosion echoing back through the jets. The data has also been turned into sound, so you can “hear” the death of the star.

The bright reverse shock emission faded away within a week and the forward shock then shone through in the ALMA band, giving astronomers a chance to see the geometry of the jet.

The optical light from the blast wave at this critical time, when the outflow has slowed just enough for all of the jet to become visible at Earth, was overshadowed by the emerging supernova from the exploded star. But ALMA’s observations, unencumbered by supernova light, enabled the astronomers to constrain the opening angle of the outflow from the jet to about 13 degrees. Understanding the morphology and duration of the outflow from the star is essential for determining the true energy of the burst. In this case, the astronomers find the jets contained as much energy as our sun puts out in a billion years.

“This is a fantastical amount of energy, but it is actually one of the least energetic events we have ever seen. Why this is so remains a mystery”, says Kate D. Alexander, graduate student at Harvard University, who led the VLA observations. “Though more than two billion light-years away, this GRB is actually the nearest such event for which we have measured the detailed properties of the outflow, thanks to the combined power of ALMA and the VLA.”

This is only the fourth gamma-ray burst with a convincing, multi-frequency detection of a reverse shock, the researchers note. The material around the collapsing star was about 3,000 times less dense than the gas surrounding stars in our galaxy, and these new ALMA observations suggest that such low-density environments are essential for producing reverse shock emission, which may explain why such signatures are so rare.

“Our rapid-response ALMA observations highlight the key role the observatory can play in following-up transients, revealing the energy source that powers them, and using them to map the physics of the universe to the dawn of the first stars,” concludes Laskar. “In particular, our study demonstrates that ALMA’s superb sensitivity and new rapid-response capabilities makes it the only facility that can routinely detect reverse shocks, allowing us to probe the nature of the relativistic jets in these energetic transients, and the engines that launch and feed them.”